1. Field of the Invention
The present invention generally relates to a control system for an internal combustion engine of cylinder injection type (also known as the direct fuel injection type engine) in which fuel is directly injected into engine cylinders to undergo combustion therein through spark ignition. More specifically, the present invention is concerned with a control system for the cylinder injection type internal combustion engine which is capable of decreasing harmful components such as nitrogen oxides NO.sub.x contained in the exhaust gas of the engine with high reliability by controlling an exhaust gas recirculation quantity (also referred to as the EGR quantity for short) with high accuracy.
2. Description of Related Art
Heretofore, the spark ignition type (or indirect fuel injection type) internal combustion engine in which fuel is injected into an intake manifold for charging a uniform air-fuel gas mixture into engine cylinders has been well known in the art. In the internal combustion engine (hereinafter also referred to simply as the engine) of this type, an air-fuel ratio sensor is provided in an exhaust pipe with a view to controlling the air-fuel ratio (also referred to simply as the A/F ratio) so that it assumes a stoichiometric air-fuel ratio (14.7). To this end, a feedback control is adopted.
By way of example, in a conventional control system for an internal combustion engine such as described in Japanese Unexamined Patent Application Publication No. 101645/1986 (JP-A-61-101645), such a technique is adopted that error or deviation of the detected air-fuel ratio from a desired value is corrected by employing a pair of air-fuel ratio sensors.
Further, in another conventional control system for an internal combustion engine such as described in Japanese Unexamined Patent Application Publication No. 75327/1988 (JP-A-63-75327), the air-fuel ratio feedback quantity is corrected upon acceleration/deceleration of the engine by storing acceleration/deceleration quantity data in combination with air-fuel ratio feedback quantity data in the form of table or map for every region where the air-fuel ratio feedback quantity is to be corrected.
In general, with the conventional internal combustion engines such as mentioned above, a relatively high output power or torque can be generated. However, the engine of this type suffers a problem that the output torque thereof changes rather remarkably as a function of the air-fuel ratio, involving thus difficulty in carrying out the control of the output torque generated by the engine.
Such being the circumstances, there has been developed a control system for a cylinder injection type engine in which fuel is directly injected into the cylinder for burning the fuel in a predetermined region within the cylinder.
By way of example, in a conventional control system for an cylinder injection type internal combustion engine such as described in Japanese Unexamined Patent Application Publication No. 312433/1996 (JP-A-8-312433), such a control scheme is adopted according to which a desired engine torque is arithmetically determined on the basis of engine operation state, and then various control quantities such as desired air-fuel ratio, desired fuel injection timing, ignition timing, EGR quantity, and so forth are determined on the basis of the desired engine torque.
For having better understanding of the principle underlying the present invention, technical background thereof will be described below in some detail. FIG. 8 is a schematic diagram showing generally an arrangement of a conventional control system for a cylinder injection type internal combustion engine known heretofore.
Referring to FIG. 8, an engine 1 constituting an intrinsically major part of the internal combustion engine system is provided with an intake pipe 1a for introducing the intake air into the engine 1 and an exhaust pipe 1b for discharging the exhaust gas resulting from the combustion of the air-fuel mixture.
An air flow sensor 2 for detecting a flow rate or quantity Qa of the intake air fed to the engine 1 as indicated by an arrow is installed at an upstream location in the intake pipe 1a. Further installed within the intake pipe 1a is a throttle valve 3 for adjusting or regulating the intake air flow rate or quantity Qa, and a throttle position sensor 4 for detecting an opening degree .theta. of the throttle valve 3 is provided in association with the throttle valve 3.
Installed at a downstream location in the intake pipe 1a, i.e., at a location immediately preceding to the engine 1 is a surge tank 5. On the other hand, an air-fuel ratio sensor 6 which may be constituted by a linear type O.sub.2 -sensor is provided in the exhaust pipe 1b for detecting an actual air-fuel (A/F) ratio F of the exhaust gas, which ratio generally lies within a range of e.g. "10" to "50".
A throttle valve actuator 7 (serving as an intake air quantity regulating means) is provided in association with the throttle valve. 3 for adjusting or regulating the throttle valve opening degree .theta.. The throttle valve actuator 7 may be comprised of, for example, a stepping or stepper motor for driving rotatively and stepwise the throttle valve 3 to thereby regulate the rate or quantity Qa of the intake air flowing through the intake pipe 1a.
Installed within each of the cylinders of the engine 1 is a spark plug 8 at which electric spark discharge takes place for igniting the air-fuel mixture within the combustion chamber of the cylinder. To this end, a distributor 9 is provided for supplying a high voltage distributively to the individual spark plugs 8 in synchronism with ignition timing.
Further provided is an ignition coil 10 which is realized in the form of a transformer having primary and secondary windings. A high voltage for the spark ignition is induced in the secondary winding of the ignition coil 10 whenever a primary current flowing through the primary winding is interrupted. The high voltage is then supplied to the distributor 9. Provided in association with the ignition coil 10 is an ignitor 11 which is constituted by a power transistor for interrupting the current flowing through the primary winding of the ignition coil 10 in conformance with the ignition timing for the individual engine cylinders.
The spark plug 8, the distributor 9, the ignition coil 10 and the ignitor 11 cooperate to constitute an ignition system or means for igniting the air-fuel mixture within the individual cylinders of the engine 1.
An ECU (Electronic Control Unit) 12 which is in charge of controlling the engine system as a whole includes a microcomputer for arithmetically determining control quantities for various actuators which are installed for the purpose of controlling combustion of in the engine 1 on the basis of information detected by various types of sensors (i.e., information concerning the operation state of the engine 1), to thereby issue driving signals indicative of the control quantities to the relevant actuators.
As the signals indicative of the various types of control quantities, there may be mentioned an intake-air flow control signal A for the throttle valve actuator 7, an ignition signal G for the ignitor 11 (ignition system), a fuel injection signal J for the fuel injection valve (i.e., injector) 13, an EGR (Exhaust Gas Recirculation) control signal E for an EGR regulating valve 17, and a purge control signal P for a purge regulating valve 26 among others.
The fuel injector 13 is mounted internally of each cylinder of the engine 1 for injecting the fuel directly into the combustion chamber defined within the cylinder. A crank angle sensor 14 for generating a crank angle signal CA is installed in association with a crank shaft which is driven rotatively by the engine 1.
For detecting a depression stroke .alpha. of an accelerator pedal manipulated by an operator or driver of a motor vehicle or the like equipped with the engine system now under consideration, an accelerator pedal stroke sensor 15 is provided in association with the accelerator pedal (not shown).
The crank angle signal CA and the accelerator pedal depression stroke signal .alpha. are inputted to the ECU 12 similarly to the other sensor signals.
As other sensors, there may be provided, for example, an intake pressure sensor for detecting the intake air pressure within the intake pipe of the engine 1, an intake-air temperature sensor for detecting the temperature of the intake air, a cooling water temperature sensor for detecting the temperature of the cooling water of the engine, etc., although they are not shown in the figure.
Additionally, there may be provided another actuator for controlling the engine 1 a high-pressure pump for injecting the fuel fed from a fuel pump 24 under high pressure although not shown.
The crank angle sensor 14 is designed to output a pulse signal corresponding to the engine rotation number or engine speed (rpm) as the crank angle signal CA and serves also as an engine rotation sensor (engine speed sensor), as is well known in the art. Further, the crank angle signal CA contains pulses having edges which correspond to reference crank angles of the plural cylinders, respectively, of the engine, wherein the reference crank angles are utilized for arithmetically determining the control timing for the engine 1.
An exhaust gas recirculation passage (hereinafter also referred to as the EGR passage) 16 is provided between the exhaust pipe 1b and the surge tank 5 for recirculating a part of the exhaust gas into the intake pipe 1a, wherein a stepping-motor-driven type EGR regulating valve 17 (constituting a part of EGR regulating means) is provided in association with the EGR passage 16 for regulating the amount or quantity of the exhaust gas recirculated to the intake pipe. This quantity is referred to also as the EGR quantity QE.
An onboard battery 20 supplies electric power to the ECU 12 by way of an ignition switch 21.
Fuel 22 for the engine 1 is contained in a fuel tank 23 to be supplied to the fuel injectors 13 by way of the fuel pump 24.
Connected to one end of the fuel tank 23 is a canister 25 containing activated charcoal for adsorbing fuel gas evaporated and dispersed from the liquid fuel contained in the fuel tank 23. The gas mentioned above will hereinafter be referred to as the evaporated gas only for convenience of the description. On the other hand, the canister 25 is communicated to the surge tank 5 by way of a purge regulating valve 26 of a solenoid-driven type.
The purge regulating valve 26 constitutes a purge means for introducing the evaporated gas generated within the fuel tank 23 into the intake pipe 1a through a purge process. Thus, when the purge regulating valve 26 is opened, the evaporated gas is introduced into the intake pipe 1a with a desired purge rate or quantity QP.
The ECU 12 serves as a control quantity arithmetic means for arithmetically determining the control quantities for the various actuators to thereby output the operation control signals A, G, J, E and P for the throttle valve actuator 7, the ignitor 11, the fuel injector 13, the EGR regulating valve 17 and the purge regulating valve 26, respectively, in dependence on the operation state of the engine 1.
FIG. 9 is a block diagram showing in detail a configuration of the ECU 12 mentioned previously by reference to FIG. 8. Referring to FIG. 9, the ECU 12 includes a microcomputer 100, a first input interface circuit 101, a second input interface circuit 102, an output interface circuit 104 and a power supply circuit 105.
The first input interface circuit 101 is so designed as to shape appropriately the crank angle signal CA to thereby generate an interrupt signal, which is then inputted to the microcomputer 100.
On the other hand, the second input interface circuit 102 is so designed as to fetch the other sensor signals (e.g. signals indicative of the intake air quantity Qa, the throttle valve opening degree .theta., the air-fuel ratio F, the accelerator pedal depression stroke .alpha., etc.) as the input signals to the microcomputer 100.
The output interface circuit 104 is designed to amplify the various actuator driving signals (e.g. the intake-air flow control signal A, the ignition signal G, the fuel injection signal J, the purge control signal P, etc.) to output the amplified signals to the throttle valve actuator 7, the ignitor 11, the fuel injector 13, etc., respectively.
The power supply circuit 105 supplies electric power from the battery 20 to the microcomputer 100.
The microcomputer 100 is comprised of a CPU (Central Processing Unit) 200, a counter 201, a timer 202, an A/D (analogue-to-digital) converter 203, a random access memory (hereinafter referred to as the RAM in abbreviation) 205, a read-only memory (hereinafter referred to as the ROM in abbreviation) 206, an output port 207 and a common bus 208.
The CPU 200 serves to arithmetically determine the control quantities for the throttle valve actuator 7 and the fuel injector 13 in dependence on the engine operation state (e.g. the accelerator pedal depression stroke a and the engine rotation number Ne indicated by the crank angle signal CA) in accordance with a predetermined program or programs.
The free-running counter 201 is designed to measure a rotation period of the engine 1 on the basis of the crank angle signal CA, while the timer 202 is employed for measuring or determining various control time points or timing as well as time durations or periods of concern.
The A/D converter 203 converts the analogue signals inputted from the various sensors to digital signals which are then inputted to the CPU 200.
The RAM 205 is used as a work memory for the CPU 200 while the ROM 206 is used for storing therein various operation programs to be executed by the CPU 200.
Various control signals (e.g. the fuel injection signal J, the ignition signal G, etc.) are outputted through the output port 207. The aforementioned individual components 201, 202, 203, 205, 206 and 207 incorporated in the microcomputer 100 are connected to the CPU 200 by way of the common bus 208.
The throttle valve opening degree .theta. is controlled in dependence on the engine operation state by using the intake-air flow control signal A. The CPU (serving as the control quantity arithmetic means) 200 incorporated in the ECU 12 arithmetically determines the fuel injection quantity on the basis of the crank angle CA (engine rotation number Ne in rpm) and the accelerator pedal depression stroke .alpha..
The fuel injector 13 is actuated in response to the fuel injection signal J having a pulse width which corresponds to the fuel injection quantity, to thereby inject a required amount or quantity of fuel into the cylinder at a predetermined timing which is derived from the crank angle signal CA. In that case, the fuel is supplied to the fuel injector 13 under a very high pressure because the fuel has to be injected directly into the cylinder.
The CPU 200 (serving as the control quantity arithmetic means) includes an injection mode changeover means for changing over the injection mode (the time point or timing at which the fuel injector 13 is actuated) in conformance with the engine operation state. As the injection modes, there may be mentioned a compression-stroke injection mode in which the fuel injection is performed in the compression stroke of the engine for realizing a stratified lean burning and a suction-stroke injection mode in which the fuel is injected in the suction stroke of the engine for realizing a lean burning or a stoichiometric feedback burning (burning of enriched fuel mixture).
Additionally, the CPU 200 is designed to issue the ignition signal G to the ignitor 11 in synchronism with the fuel injection timing. The ignitor 11 operates in response to the ignition signal G to electrically energize the ignition coil 10 for producing the electric spark discharge at the spark plug 8 in a predetermined timing by way of the distributor 9.
Next, description will be directed to the operation of the conventional control system for the cylinder injection type internal combustion engine of the structure described above by reference to FIGS. 8 and 9.
When the crank angle signal CA is inputted to the ECU 12A, an interrupt signal is issued through the first input interface circuit 101 in response to a pulse edge of the crank angle signal CA.
In response to the interrupt signal, the CPU 200 reads out the content or value of the counter 201 to thereby determine arithmetically the rotation period of the engine 1 on the basis of a difference between a current counter value and a preceding one, the rotation period as determined being then stored in the RAM 205. Further, the CPU 200 arithmetically determines the engine rotation number or engine speed Ne (rpm) on the basis of the above-mentioned rotation period and the measured time or period corresponding to a predetermined crank angle which can be derived from the crank angle signal CA.
On the other hand, through the second input interface circuit 102, the analogue sensor signals such as the signals indicative of the accelerator pedal depression stroke a and others are fetched to be supplied to the CPU 200 after having been converted to the corresponding digital signals by the A/D converter 203.
The control quantity arithmetic means realized by the CPU 200 arithmetically determines various control parameters or quantities on the basis of the sensor information indicative of the engine operation states to thereby output driving signals corresponding to the control quantities to the relevant actuators mentioned previously by way of the output port 207 and the output interface circuit 104.
By way of example, the CPU 200 incorporated in the ECU 12 arithmetically determines a desired opening degree of the throttle valve (hereinafter referred to as the desired throttle valve opening degree) on the basis of the sensor signal indicative of the accelerator pedal depression stroke a to thereby output the intake-air flow control signal A which indicates the desired throttle valve opening degree. In response to this signal A, the throttle valve actuator 7 is so driven that the actual throttle valve opening degree detected by the throttle position sensor 4 coincides with the above-mentioned desired throttle valve opening degree.
Further, the CPU 200 arithmetically determines a desired fuel injection quantity to thereby output the fuel injection signal J which indicates the desired fuel injection quantity. In response thereto, the fuel injector 13 is actuated in the predetermined timing based on the crank angle signal CA to inject the fuel directly into the cylinder of the engine 1 so that the fuel as injected coincide with the desired fuel injection quantity.
Besides, the CPU 200 arithmetically determines a desired ignition timing to output the ignition signal G indicative of the desired ignition timing for thereby driving the ignitor 11 at a predetermined timing in synchronism with the fuel injection timing.
As a result of this, the primary current of the ignition coil 10 is interrupted in response to the ignition signal G, whereby the high voltage induced in the secondary winding of the ignition coil 10 is applied to the spark plug 8 through the distributor 9. Thus, electric discharge occurs at the spark plug 8 at the predetermined ignition timing to generate the spark for ignition.
Furthermore, the EGR regulating valve 17 is driven in response to the EGR control signal E conforming to the engine operation state, whereby the EGR quantity QE is controlled optimally. Additionally, feedback control is performed on the fuel quantity on the basis of the air-fuel ratio F detected by the air-fuel ratio sensor 6 so that the actual air-fuel ratio coincides with the desired air-fuel ratio.
Next, referring to FIGS. 10 to 12 together with FIGS. 8 and 9, description will be made of operation of the conventional control system for the cylinder injection type internal combustion engine in the concrete. FIG. 10 is a flow chart for illustrating typical operations of the ECU 12.
FIG. 11 is a graphic view for illustrating two-dimensionally preset regions of desired EGR ratios (%) corresponding or equivalent to desired EGR quantities Eo in the compression-stroke injection mode (stratified lean burn mode). In FIG. 11, the engine rotation number or engine speed Ne (rpm) is taken along the abscissa with the accelerator pedal depression stroke .alpha. (%) indicative of the engine load being taken along the ordinate, wherein the desired EGR ratio assumes a maximum value which is greater than e.g. 40% or more when the engine rotation number Ne and the accelerator pedal depression stroke .alpha. are at middle levels, respectively.
FIG. 12 is a view for graphically illustrating a relation between the desired EGR quantity Eo in liter/sec and an opening degree .theta.E of the EGR regulating valve 17 in terms of the number of times the EGR regulating valve 17 is driven stepwise.
During operation of the engine 1, signals outputted from the various types of sensors and indicating the engine operation state are inputted to the ECU 12, which executes a processing illustrated in FIG. 10 on the basis of the crank angle signal CA at every predetermined ignition timing or every predetermined time.
Referring to FIG. 10, in a step S2, the ECU 12 determines discriminatively the engine operation state on the basis of the engine rotation number Ne, the accelerator pedal depression stroke .alpha., etc. in a step S1, and then determines arithmetically the desired EGR quantity Eo by referencing the two-dimensional data map of the EGR ratios (see FIG. 11).
In succession, the ECU 12 generates the EGR control signal E on the basis of the characteristic data of the EGR valve opening degree .theta.E (see FIG. 12) so that the EGR quantity QE coincides with the desired EGR quantity Eo. Operation of the EGR regulating valve 17 is controlled by the EGR control signal E (step S3).
Subsequently, the ECU 12 arithmetically determines the desired exhaust gas air-fuel ratio A/Fo conforming to the current engine operation state (step S4).
Furthermore, in a step S5, the ECU 12 arithmetically determines combustion parameters (the desired fuel injection quantity, the desired fuel injection timing and the desired ignition timing) on the basis of the desired air-fuel ratio A/Fo and the intake air quantity Qa to generate the fuel injection signal J and the ignition signal G which conform to the above-mentioned control quantities, for thereby controlling operations of the fuel injector 13 and the ignitor 11, respectively.
In succession, the ECU 12 detects the air-fuel ratio F of the exhaust gas resulting from the actual combustion on the basis of the output signal of the air-fuel ratio sensor 6 in a step S6, while making decision as to whether or not the engine is in the compression-stroke injection mode (stratified lean burn mode) in a step S7.
When decision is made that the engine is in the compression-stroke injection mode (i.e., when the step S7 results in affirmation "YES"), the fuel injection control in the compression-stroke injection mode (not shown) is executed, whereon the processing routine illustrated in FIG. 10 makes return to the starting state "START" (i.e., the state ready for starting again the processing routine).
On the other hand, when it is decided that the engine operation is not in the compression-stroke injection mode (i.e., when the decision step results in negation "NO"), this means that the engine operates in the suction-stroke injection mode (stoichiometric burn mode). Accordingly, the ECU 12 executes the feedback control so that the actual air-fuel ratio F coincides with the desired exhaust gas air-fuel ratio A/Fo in a step S8), whereon the processing routine shown in FIG. 10 returns to the starting state "START".
In this manner, a predetermined amount or quantity of fuel is injected in each cylinder of the engine 1, and an air-fuel mixture containing the fuel is fired or ignited within the cylinder at a predetermined timing, whereby optimum operation of the engine 1 can be ensured.
However, in the case of the cylinder injection type engine 1, combustion or burning takes place with a very large air-fuel ratio (for very lean burning), differing from the conventional indirect injection type engine in which the fuel is injected into the intake manifold. Accordingly, the exhaust gas recirculated through the EGR passage 16 contains a relatively large amount of fresh air in addition to the intrinsic exhaust gas.
Consequently, the air-fuel ratio prevailing within the cylinder after the combustion performed with the fuel injection quantity set so that the air-fuel ratio F detected actually becomes equal to the desired exhaust gas air-fuel ratio A/Fo in the state where the EGR regulating valve 17 is fully closed (i.e., the EGR quantity QE is zero) differs from the air-fuel ratio prevailing within the cylinder after the combustion performed by setting the fuel injection quantity similarly but in the state where the EGR regulating valve 17 is opened, because the EGR quantity QE contains the fresh air.
More specifically, in the indirect fuel injection type engine where the fuel is injected into the intake manifold 1a, the exhaust gas for the EGR is exactly the gas resulting from the burning or combustion in the intrinsic sense. However, in the case of the engine 1 of the cylinder injection type, the gas which undergoes the recirculation contains not only the exhaust gas in the intrinsic sense (i.e., gas resulting from the combustion) but also the fresh air which has made no contribution to the burning in the stratified combustion mode.
The relation described above can be represented by the undermentioned expression (1) in the case of the intake manifold injection type engine. EQU A/Fo=Qan/Fj=A/Fr (1)
where A/Fo represents the desired exhaust gas air-fuel ratio, Qan represents the amount or quantity of fresh air charged into the engine 1, Fj represents the quantity of fuel injected into the engine 1 and A/Fr represents the actual or real air-fuel ratio.
By contrast, in the case of the cylinder injection type engine, the recirculated exhaust gas containing the fresh air is charged into the engine 1. Consequently, the real air-fuel ratio A/Fr appearing in the above expression (1) is modified as follows: EQU A/Fr=(Qan+QEn)/Fj (2)
where QEn represents the amount or quantity of fresh air which is contained in the exhaust gas (i.e., the quantity of fresh air which has not contributed to the combustion in the stratified lean burn).
As can be seen from the above expression (2), in the case of the cylinder injection type engine 1, the content of the fresh air (QEn) contributes to increase of the air-fuel ratio A/Fr. For this reason, the inconvenience mentioned previously is incurred.
Furthermore, it is noted that since error contained in the output of the air-fuel ratio sensor 6 generally increases in the lean region where the air-fuel ratio is large A/F&gt;30), the reliability of the air-fuel ratio control in the state where the EGR operation is being validated is further degraded.
Furthermore, it is to be added that upon engine accelerating operation, the purge regulating valve 26 is opened for feeding the evaporated gas adsorbed in the canister 25 into the surge tank 5 to increase materially the fuel injection quantity for thereby enriching the air-fuel mixture for combustion. Thus, the reliability of the air-fuel ratio control will be degraded during such purge process.
Next, referring to FIGS. 13 and 14, description will turn to the purge process in the control system for the cylinder injection type internal combustion engine known heretofore. FIG. 13 is a flow chart for illustrating a processing procedure for operating the purge regulating valve 26, and FIG. 14 is a view for graphically illustrating a relation between a purge valve driving duty ratio Dp and an actual purge quantity QP.
Referring to FIG. 13, the ECU 12 makes decision as to whether or not the condition for the purge operation is met by deciding whether or not a predetermined time has lapsed from the time point the warm-up operation of the engine 1 was started (step S11).
Unless the condition for the purge operation is met (i.e., when the step S11 is "NO"), the processing routine shown in FIG. 13 makes return to the starting state "START" without executing any processing. On the contrary, when the condition for the purge operation is met (i.e., when the decision step S11 results in "YES"), a desired purge quantity QPo is arithmetically determined in a step S12 by referencing the two-dimensional data map prepared previously on the basis of the engine rotation number Ne and the accelerator pedal depression stroke .alpha. (equivalent to the engine load).
In succession, in order to determine the required purge quantity QP, the characteristic data of the purge valve driving duty ratio Dp such as illustrated in FIG. 14 is referenced. More specifically, the ECU 12 determines the driving or operation duty ratio Dp for the purge regulating valve 26 on the basis of the relation between the desired purge quantity QPo and the purge valve driving duty ratio Dp.
Subsequently, the ECU 12 generates the purge control signal P for making available the requisite purge quantity (requisite quantity of purged fuel) QP by driving or operating the purge regulating valve 26 in a step S14, whereon the processing routine illustrated in FIG. 13 returns to the starting state "START".
As is apparent from the foregoing description, the control system for the cylinder injection type internal combustion engine is not in the position to eliminate or remove sufficiently the harmful components such as NO.sub.x contained in the exhaust gas because the actual EGR quantity QE can not be controlled to an appropriate value conformable to the load of the engine 1, suffering thus a problem that the combustion (or burning) behavior of the engine (and hence drivability of the motor vehicle) is degraded with harmful components such as NO.sub.x contained in the exhaust gas being increased.
Besides, because error or deviation is brought about by the air-fuel ratio sensor 6 due to the difference between the compression-stroke injection mode (stratified lean burn mode) and the suction-stroke injection mode (stoichiometric burn mode) as described above, there arises a problem that the combustion state or behavior and the exhaust gas quality will become worse.
Besides, in the case where the purge quantity QP of the evaporated gas is controlled by driving correspondingly the purge regulating valve 26, error or deviation occurs in the air-fuel ratio control, incurring degradation of the combustion behavior and the exhaust gas quality.